1. Field of the Invention
The present invention relates to a sheet processing apparatus in which booklet-like bundles of folded sheets are stacked and an image forming apparatus provided with such a sheet processing apparatus in its main body. In particular, the present invention relates to a sheet processing apparatus in which the folded portion of a succeeding sheet bundle is prevented from entering an open portion opposed to the folded portion of a preceding sheet bundle.
2. Description of the Related Art
In some conventional image forming apparatuses for forming an image on a sheet, a sheet processing apparatus is equipped as a component of the apparatus. The sheet processing apparatus bundles sheets onto which images have been formed in the main body of the apparatus, binds the bundle and then folds it into a booklet form. In such a sheet processing apparatus, sheets are sequentially received on a tray to align edges of sheets to make a sheet bundle, binds the sheet bundle near a central portion in an edge part of the bundle, pushes the central portion by a pushing member to thrust the sheet bundle into the nip of a pair of rollers, and folds the sheet bundle by the pair of rollers while conveying it. In addition, Japanese Patent Application Laid-Open No. 2003-182928 discloses a sheet processing apparatus that performs a fold line processing to form a neat fold line.
The operation of such a conventional sheet processing apparatus will be described with reference to FIGS. 24 to 26. After the sheet processing apparatus aligns edges of multiple sheets in the sheet bundle on a collecting portion 70, it staples the central portion, with respect to the conveyance direction, of the sheets with a wire(s). Subsequently, the apparatus pushes the central portion of the sheet bundle P by a center folding plate 82 to thrust the sheet bundle into the nip between a first folding roller pair 83, 84. The first folding roller pair 83, 84 folds the sheet bundle while conveying it, and then temporarily stops it.
Then fold line enhancing process is performed using second folding rollers that are different from the first pair of folding rollers 83, 84 by holding the folded portion between a second folding roller or second folding rollers 85, and moving a holding member 851 that supports the second folding rollers 85 along the fold line (i.e. in the direction perpendicular to the conveyance direction). Thus, a center-folded sheet bundle (hereinafter referred to as “folded sheet bundle”) that is folded at its center is obtained. Subsequently, the operation of the first folding roller pair 83, 84 is restarted, so that the sheet bundle is conveyed and discharged onto a tray 63.
In the conventional sheet processing apparatus, the sheet bundles must be stacked on the tray 63 in such a way that the trailing edge of the folded sheet bundle is positioned outside the operation region of the holding member 851 i.e., downstream with respect to the conveyance direction, so as not to interfere with the moving holding member 851 of the second folding roller 85. However, keeping such a positional relationship leads to a broad spacing between the trailing edge of the sheet bundles stacked on the tray 63 and the first folding roller pair 83, 84.
Accordingly, in some cases, the folded portion of the succeeding folded sheet bundle that is being discharged droops down and gets stuck with the rear edge of the preceding folded sheet bundle stacked on the tray 63. Consequently, the succeeding folded sheet bundle may abut the preceding folded sheet bundle or get into the open trailing end of the preceding folded sheet bundle, so that jamming of the folded sheet bundles, stack failure or bending of sheets sometimes occur.
The smaller the thickness of the sheets that constitute the folded sheet bundle is, the lower the stiffness of the sheets is, and the smaller the number of the folded sheets is, the more likely the folded sheet bundle droops down, and the more prominent the above-described problem is.
The above-described phenomenon also occurs in the case of a type of sheet processing apparatus in which the holding member 851 of the second folding roller 85 is not provided and the folded sheet bundle is delivered onto the tray 63 by the first folding roller pair 83, 84 without undergoing a fold line enhancing process.
As per the above discussion, in conventional sheet processing apparatuses, improper stacking of the folded sheet bundles sometimes occurs.
Provision of a sheet processing apparatus that suffers from improper stacking of folded sheet bundles in an image forming apparatus leads to a low image processing efficiency of the image forming apparatus as a whole.